The data conveyed by an asynchronous frame are generally in the form of 10-bit characters, each comprising an 8-data bit chain preceded by a start bit and followed by a stop bit. These two framing bits always have opposite values, either 0 and 1 or 1 and 0. It follows that a data character cannot comprise a string of 10 bits of the same value. Two successive frames can therefore be separated by an idle interval during which a word comprising 10 bits of the same value, for example 1 bit, is generated.
When a UART circuit attempts to hot plug onto an asynchronous data bus and it is not synchronized with the bus, it must detect the idle interval before detecting the start bit of the first character received.
Furthermore, in asynchronous data transmissions, the receiver does not receive the transmitter's clock signal. The respective clocks of the transmitter and the receiver must have a deviation in relation to each other that does not exceed a certain value, so that the data can be received correctly.
To increase the possibilities of transferring asynchronous data between devices having rather inaccurate clock circuits and likely to have considerable clock drifts in relation to each other, data transmission protocols have recently been developed that enable a receiver to set its clock signal to that of a transmitter through the sending of a synchronization character by the latter. These protocols are, as a result, less demanding in terms of the receiver's clock signal deviation in relation to that of the transmitter. In the text below, local clock signal shall mean the receiver's clock signal, and reference clock signal shall mean the clock signal present in a synchronization character.
As an example, FIG. 1 represents an asynchronous frame according to the LIN protocol (Local Interconnect Network). This frame starts with a break character BRK comprising a string of 0 bits and ending with a last bit equal to 1 (extra bit). The string of 0 bits has a minimum length of 13 bits and is deemed to be detected when at least 11 0 bits are detected. The number of 11 bits is chosen conventionally and allows a deviation on the order of 15% to be tolerated between the local clock signal and the reference clock signal. The frame continues with a synchronization character SYNC followed by one or more data characters CH1, CH2, . . . CHN. The first data character CH1 is commonly used as the identification field of the frame recipient.
However, in the LIN protocol as in other protocols, the transmission of an idle interval between two frames is not compulsory. It follows that the receiver may take considerable time to hot plug onto an asynchronous data bus since it must wait for the occurrence of this idle interval. Thus, it might not take into account data characters that were intended for it if they have been transmitted before it detects an idle interval.